Despite evidence which shows most experimentation with alcohol and tobacco occurs in the company of friends, little work has examined the role adolescent friends play in intervening to stop a peer from engaging in these risk behaviors. The current research plan extends previous cross-sectional studies which suggest adolescent friends can have a positive influence on one another's risk behaviors by exploring how adolescents'willingness to intervene changes over time. Using longitudinal latent variable mixture modeling techniques, the proposed study examines changes in adolescents'self-reported responses to hypothetical vignettes in which they have knowledge of, or witness, friends using ATOD. Data consist of self-reports from 838 adolescents, ages 10 to 20, who responded to survey questionnaires over a period of three years. This proposed study has four specific aims: (a) explore patterns in adolescents'responses (e.g., ignore, intervene) to friends'ATOD use, (b) model developmental change in adolescents'responses to friends'ATOD use, (c) examine gender differences in the probability of intervening and/or ignoring the risk behaviors of friends over time, and (d) identify the developmental, social, and behavioral factors that affect change in the way that adolescents respond to the ATOD risk behaviors of friends. It is anticipated that the results of this study will have serious implications for the prevention of ATOD risk behaviors in adolescents. This study will provide prevention scientists with a more complete picture of the role of friends in ATOD use/ prevention and basic developmental knowledge about how friends can be used as allies in prevention. These findings may, ultimately, be used to inform the development of new prevention curricula. PUBLIC HEALTH RELEVANCE: The proposed research seeks to understand how young people's willingness to intervene in the ATOD risk behaviors of friends changes over time and to identify those developmental, social, and behavioral factors that predict this change. This information will then be used to design developmentally appropriate prevention programs that emphasize the role of friends in ATOD risk reduction.